The present invention relates to a substrate for a photoelectric conversion device and a method of manufacturing the same, and further to a photoelectric conversion device using this substrate.
In a thin film photoelectric conversion device, a transparent conductor on which a transparent conductive film of tin oxide, ITO, or the like is formed on a glass surface is used as a substrate. As the transparent conductive film, a film containing tin oxide as a main component has been used in many cases. A thin film photoelectric conversion device using thin film silicon as a photovoltaic material has been receiving attention due to the low energy cost required for its manufacture or the like.
Generally, a thin film silicon-based photoelectric conversion device includes an undercoating film, a transparent conductive film, thin film silicon, and a metal film, which are formed sequentially on the surface of a glass sheet. As the transparent conductive film, a tin oxide film has been used in many cases, which is formed by a method accompanied by a pyrolytic oxidation reaction of raw materials, such as a CVD method or the like. The undercoating film is provided for preventing an alkaline component such as sodium or the like contained in the glass sheet from diffusing into the transparent conductive film, thus preventing a decrease in electrical characteristics (i.e. the increase in resistance) of the transparent conductive film. As the undercoating film, a transparent thin film such as a silicon oxide film or the like is used.
The transparent conductive film of the thin film photoelectric conversion device is required to have a high transmittance (i.e. to introduce a larger quantity of light into a photovoltaic layer) and a low resistance (i.e. to reduce the loss in leading out generated electricity). It has been known that to provide the surface of the transparent conductive film with proper roughness is effective in trapping light in the photovoltaic layer. Therefore, in a substrate for a thin film photoelectric conversion device, it is required that the transmittance is high and a haze ratio reflecting the surface roughness also is high to some degree.
Methods for providing the surface of the transparent conductive film with roughness include, a method of forming the surface of an undercoating film to have roughness. As a method of manufacturing a silicon oxide film having a surface with roughness, for instance, JP 60-175465 A discloses a method using a processing liquid obtained by adding boric acid to an aqueous solution of hexafluorosilicic acid saturated with silicon oxide. Further, JP 62-44573 A discloses a method using gaseous molecules containing silicon atoms and an oxidizing gas. In both these methods, silicon oxide particles produced in the reactions are allowed to be contained in a silicon oxide coating film.
However, the above-mentioned conventional methods have not enabled a substrate for a photoelectric conversion device effective for the light trapping in a photovoltaic layer to be manufactured by industrial mass-production. Since the silicon oxide film having a surface with roughness is formed by allowing silicon oxide particles produced in the reactions to be contained in the silicon oxide coating film, a film formation reaction and a particle production reaction must be controlled simultaneously. Therefore, it is difficult to carry out stable manufacturing continuously.
The present invention is intended to provide a substrate for a photoelectric conversion device, such as a photovoltaic device, that is effective for light trapping in a photoelectric conversion layer and can be manufactured by industrial mass-production, and a method of manufacturing the same. Further, the present invention is intended to improve photoelectric conversion characteristics of a photoelectric conversion device by using this substrate.
In order to achieve the aforementioned objects, a substrate for a photoelectric conversion device according to the present invention includes a first undercoating film containing, as a main component, at least one selected from tin oxide, titanium oxide, indium oxide, and zinc oxide, a second undercoating film, and a conductive film, which are formed on a glass sheet containing an alkaline component in this order. In the first undercoating film, holes are formed.
According to the above-mentioned configuration, a substrate can be provided that is suitable for a thin film photoelectric conversion device and can be manufactured by industrial mass-production.
In the substrate for a photoelectric conversion device, it is preferable that the first undercoating film has at least two holes per square micron. Further, in the substrate for a photoelectric conversion device, it is preferable that the first undercoating film has a thickness in the range between 10 nm and 100 nm. In the substrate for a photoelectric conversion device, it also is preferable that the glass sheet is a float glass sheet obtained by a float glass process and the first undercoating film, the second undercoating film, and the conductive film are formed on the top surface of the float glass sheet. In this case, the top surface denotes the surface opposite to the surface (the bottom surface) in contact with molten tin in a float bath during the formation by the float glass process.
In order to achieve the above-mentioned object, a method of manufacturing a substrate for a photoelectric conversion device according to the present invention includes forming a first undercoating film, a second undercoating film, and a conductive film in this order on a glass sheet containing an alkaline component, or on a glass ribbon in a manufacturing process of the glass sheet. In the method, the first undercoating film is formed to include holes by a pyrolytic oxidation reaction of a coating-film forming material containing chlorine on the glass sheet or the glass ribbon having a temperature of at least 600xc2x0 C.
In the manufacturing method, it is surmised that sodium chloride produced in the first undercoating film by the reaction between sodium in the glass sheet and chlorine in the material is lost from the film to form the holes in the first undercoating film. Thus, a substrate suitable for a thin film photoelectric conversion device can be manufactured by industrial mass-production.
The present invention also provides a photoelectric conversion device using the above-mentioned substrate. In this photoelectric conversion device, at least one photoelectric conversion unit and a back electrode are stacked on the conductive film of the substrate for a photoelectric conversion device in this order. This photoelectric conversion device is used with its glass sheet side positioned as the light incident side.